Electrolysis Exercise

Construction:

To build an electrolysis experiment you need four basic items. A power source, water with a suitable electrolyte, electrodes, and wires to connect everything. A 9v battery is a good power source since its voltage is high enough to generate visible bubbling, but is a small, compact, easy-to-use package. For the water I used baking soda (sodium bicarbonate). An acid may also be used (e.g. white vinegar). Do not use table salt (sodium chloride) since the reaction will produce chlorine gas. Any conductive material may be used for the electrodes---metals are common. I used graphite (thick pencil lead) for the electrodes due to their relative inefficiency (explained below). Finally, the way the apparatus is wired up is up to you. Alligator clips make assembly easy. I soldered alligator clip leads to a 9v battery clip to allow for simple installation of the battery, and protecting against shorts.

Design and Safety:

Electrolysis of water produces hydrogen and oxygen gas. These gasses in combination are combustible, so in any electrolysis there is a possibility of explosion. However, it is easy to design the experiment so that the chances of such an event are vanishingly small. The risk of combustion grows with the concentration of the gasses produced---less gas, the less likely to accidently ignite it, and also the smaller (less energetic) the resulting explosion would be. We want to limit the rate of production of the gasses. This rate is a function of current flowing through the water. Thus, to limit gas production we limit current. Current is a function of voltage and resistance, and so the apparatus is designed with a low voltage (9v) and a high resistance. The voltage is supplied by the battery, so the design will mostly affect resistance. Resistance is raised in three primary ways: (1) Graphite electrodes. These electrodes are less efficient than a metal like copper. (2) Distance between electrodes. The current must pass through the water solution separating the electrodes. The greater the distance, the greater resistance. (3) Dissolved electrolyte. Pure water is not a good conductor, and so an electrolyte is typically added. Adding less electrolyte will retain a higher resistance.